An electromagnetic fuel injector comprises a cylindrical tubular accommodation body presenting a central feeding channel, which performs the function of fuel pipe and ends with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a needle, which is rigidly connected to a mobile keeper of the electromagnetic actuator to be displaced by the bias of the electromagnetic actuator itself between a closing position and an opening position of the injection nozzle against the bias of a closing spring which tends to maintain the needle in the closing position. The needle ends with a shutting head, which in the closing position is pushed by the closing spring against the valve seat of the injection valve to prevent the output of fuel. Generally, the shutting head is arranged inside the fuel pipe and consequently, to pass from the closing position to the opening position of the injection valve, the shutting head is displaced in a sense contrary to the feeding sense of the fuel remaining within the fuel pipe; these fuel injectors are named inward opening fuel injectors.
Inward opening fuel injectors cannot ensure a high precision and a high stability in the fuel injection direction and thus are not suitable for being used in the so-called “spray-guided” engines which use a stratified combustion, in which the fuel must be injected with a very high precision near the spark plug; indeed, in this type of application an error of less than one millimeter in the fuel flow direction may wet the spark plug electrodes and thus seriously compromise combustion.
In order to obtain a high precision and a high stability in the fuel injection direction, outward opening fuel injectors are used, in which the shutting head presents a truncated-cone shape, is arranged outside the fuel pipe, is pushed by a closing spring against the valve seat of the injection valve itself with a sense contrary to the feeding sense of the fuel, and is consequently displaced from the closing position to the opening position in a sense agreeing with the feeding sense of the fuel.
In order to obtain optimal features of the fuel injection, the hydraulic sealing diameter of the truncated-cone shaped shutting head is high and in the order of 3.5-4 mm instead of 1.3-1.5 mm of a head of the standard ball shutter. When the engine is running, high-pressure fuel (about 150-200 bars) is present inside the feeding pipe, which fuel generates a hydraulic opening thrust of considerable proportions on the shutting head by effect of the large hydraulic sealing area; such hydraulic opening thrust on the shutting head must be contrasted by the closing force of the closing spring which must be consequently dimensioned to generate a considerable elastic closing force. Consequently, also the electromagnet must be dimensioned to be capable of generating a considerable electromagnetic opening force higher than the elastic closing force of the closing spring to allow to start the engine; indeed, when the engine has started, the elastic closing force generated by the closing spring is contrasted by the hydraulic opening thrust generated by the pressurised fuel, while the hydraulic opening thrust generated by the pressurised fuel is generally absent when starting the engine (the high pressure fuel pump is mechanically actuated by the crankshaft and thus static before the engine is started).
Dimensioning both the closing spring and the electromagnet for respectively generating an elastic force and an electromagnetic force of high intensity implies high production costs and heavy weights which determine considerable mechanical and magnetic inertia with consequent worsening of the dynamic performances of the injector (i.e. reduction of the actuation speed); the worsening of the dynamic performances of the injector is particularly negative, because it prevents actuating the injector for short injections and thus prevents the performance of short pilot injections before the main injection.
In order to solve the aforesaid drawbacks, it has been suggested to replace the traditional electromagnetic actuator with a piezoelectric actuator, which is adapted to generate very high piezoelectric forces with very short actuation times. However, a piezoelectric actuator is currently very costly and difficult to make.